The Dirichlet distribution is the conjugate prior of the Multinomial distribution, and hence can be used to implement a PPR-based stopping rule, which is similar in principle to the multi-variate PPR given in Appendix C in [1]}. In specific, the PDF of the Dirichlet distribution is given as
\(pdf(x) = \frac{\prod _{i=1}^{K}x_i^{\alpha _i-1}}{B(\alpha )}, \\B(\alpha ) = \frac{\prod _i \Gamma (\alpha _i)}{\Gamma (\sum _i \alpha _i)}.\) 
